Propecia's 1 mg dose of finasteride can effectively lower DHT levels in the scalp by as much as 60% when taken daily. It is DHT that shrinks or miniaturizes the hair follicle, which eventually leads to baldness. This 60% reduction in DHT has proven to stop the progression of hair loss in 86% of men taking the drug during clinical trials. 65% of trial participants had what was considered a substantial increase of hair growth.

Men’s Rogaine Extra Strength Solution is the liquid version of our top pick. It didn’t make our final cut because it includes propylene glycol, which causes irritation in roughly one-third of its users. With that said, Dr. Wolfeld finds that it can be even more effective in practical daily use. In his experience, “the solution can penetrate and get into your scalp a little bit better” than the foam — especially if you’re not taking the time and effort to apply the foam correctly. This seems crazy to us since the foam so quickly dissolved into a liquid in our tests, but if you’re worried, try a one-month supply of the liquid and make the switch to foam if you notice any irritation.

Adam Friedman, a program director at The George Washington University School of Medicine & Health Sciences, is on the same page as Romanowski. "While it does have antimicrobial properties which may be useful in terms of fighting off bacterial or fungal overgrowth on the scalp that can lead to hair-damaging inflammation, there is zero evidence [showing] it is helpful for hair growth," he says, adding that some people can actually be allergic to castor oil and experience inflammation, ultimately doing more damage than good to the scalp. "To propose that castor oil accelerates hair growth, a tightly regulated process (one centimeter per month) for which FDA approved medications for hair loss do not impact, is ridiculous," he states.

In either sex, hair loss from androgenetic alopecia occurs because of a genetically determined shortening of anagen, a hair's growing phase, and a lengthening of the time between the shedding of a hair and the start of a new anagen phase. (See "Life cycle of a hair.") That means it takes longer for hair to start growing back after it is shed in the course of the normal growth cycle. The hair follicle itself also changes, shrinking and producing a shorter, thinner hair shaft — a process called "follicular miniaturization." As a result, thicker, pigmented, longer-lived "terminal" hairs are replaced by shorter, thinner, non-pigmented hairs called "vellus."